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Brackish Tank Setup - Written by Sploke

Setting up a Brackish-water aquarium by Sploke

So you’re interested in maybe setting up a brackish tank? Freshwater is getting too overdone for you, but marine just seems to daunting? Just looking for something new, or a specific species? Bought a fish and found out that it needs brackish water, and looking to convert? The aim of this article is to clear up some of the questions surrounding a brackish habitat, and give folks a good jumping-off place to start doing more detailed research into what might be of particular interest.

To begin, I’ll define a few basic terms that you’ll hear quite often that you’ll need to know in order to understand a brackish system.

Specific gravity (of water) – this is the measure of the density of the tank water. Pure water at 4 degrees F will have a SG of 1.000. As you add something into solution (in our case, salt) the specific gravity will increase. For reference, accepted SG for a reef system is between 1.024 and 1.026.

Salinity – This is another measure of density of water, although it references salt specifically (you can use any number of substances to get water to a certain SG, although hopefully you’re only using salt in your aquarium).

Marine mix – salt specifically formulated to make an artificial approximation of natural seawater. Note: marine mix is very different from “aquarium salt”. This will be covered later.

Hydrometer – tool for measuring salinity/specific gravity. There are several types – some you fill with water and have a floating needle that gives you a reading, others look like glass tubes with a scale inside that you float in the water. Most are calibrated to work around 70 degrees F.

Refractometer – another tool for measuring density of water, generally much more accurate than a hydrometer, but proportionally more expensive. It actually works by measuring the refractive index of the liquid in question, which corresponds to a specific gravity.

First off, Webster’s dictionary defines brackish as “slightly salty”. This is a decent starting place, although we will get much more specific. It is clear, however, that there can be a pretty broad range of salinity that can be considered brackish. Technically, any specific gravity from 1.001 up to about 1.024 can be considered brackish. Brackish habitats are found all over the world. Wherever a freshwater stream or river forms an estuary meeting the ocean or a salt sea, you get brackish water. Between the downstream flow of the river and the ebb and flow of waves and tides of the sea, seawater will mix with freshwater at the mouth of the river, and sometimes upriver as far as a mile or more. This creates a gradient from pure freshwater to pure seawater over the course of the end of the river.

Standard Equipment

Setting up a brackish tank is almost identical to setting up a freshwater tank. As long as you understand cycling and the nitrogen cycle, you can do it. All you need is a fish tank, some type of filter (reverse flow undergravel, canister, or HOB will all work equally well, it just depends on personal preference), sufficient lighting to see the fish, marine salt, and some cover/structure within the tank. Just like any other aquarium, a brackish tank needs both mechanical and biological filtration. How you accomplish this is up to you. Cycle the tank and you’re ready to go. If you have further questions about how to cycle a tank, I would suggest reading the cycling article. The process is exactly the same with a brackish, tank, just mix salt to the desired SG before starting the process.

Some people will recommend using a protein skimmer on a brackish tank. I have personally never tried it, so I cannot attest firsthand to their effectiveness. I know that on the basis of how a protein skimmer works, I wouldn’t expect it to be nearly as effective as when used on a marine tank, but some people opt for them regardless. I will leave that research and decision up to you.

One mistake that many people make when setting up a brackish habitat is using aquarium salt, or even table salt. This is not equivalent to using true marine mix. Aquarium salt is plain old NaCl, sometimes with iodide added. Natural seawater is a far more complex solution, with many other trace elements that are necessary for the long-term well-being of your aquarium. For a list of the elemental makeup of natural seawater, look here.

Another common question about new brackish tanks is the usage of liverock and live sand. As far as I’ve seen, there is no brackish water equivalent. From research, it looks like the microorganisms that make these media “live” only tolerate SG down to about 1.020 or so. So, for most brackish environments, these two items are not plausible options as far as effective filtration elements. You can still use live rock or live sand for the visual appeal of course, but you won’t get the benefits of the microorganisms native to these media. If you are planning on using either, you will have to cure them due to the die-off that will occur due to keeping them in unsuitable conditions to maintain the “live” part.

How salty should I make it?

Well, that depends on what you want to keep. Many brackish water fish will do fine in a SG as low as 1.005. This is considered low-end brackish. Many common fish such as most gobies and figure-8 puffers will do fine for the entirety of their lives at the low end of the spectrum. This is a bonus if you want to keep plants as well, since most plants that tolerate brackish only do so at the low end of the spectrum. Other species, such as Ceylon puffers and Columbian sharks, will require low to mid end brackish as juveniles and move on to high-end or even full marine as adults. The particular requirements of your specific livestock will help you determine what your target salinity should be. Research and double-check your information well before starting the tank to ensure that your species are compatible and that you are willing and able to provide the proper habitat.

One more note – similar to marine tanks, you will want to mix the brackish water in a bucket or storage tub before adding it to the tank. DO NOT fill the tank with water, then dump in the salt. Premixing it accomplishes two things – you can test the temperature and salinity and adjust as necessary before adding it to the tank, and it saves the fish a lot of stress. Dumping undissolved salt into the tank can possibly burn the fish if the fish comes in contact with the grains, and causes undue stress that is easily avoided. Another tip to remember is, always add salt to water, not water to salt. If you add water to salt, you end up with very high concentrations of ions while the container is filling. At these high concentrations, some elements will react and precipitate out, leaving them unavailable for use in the water column. Filling up your mixing container then adding the marine salt will ensure that this does not happen.

What if I already have the fish?

This is a relatively common problem – the well-meaning aquarist buys a “freshwater” fish from Wal-Mart or a large chain pet store, and after doing some research, finds out that the poor fish belongs in brackish water! Don’t panic. The saving grace is that, in an estuarine environment, these fish have adapted to a certain amount of adversity in their conditions, so usually a short stint in plain freshwater will not kill them. You have made the important first step however, in identifying the requirements for the fish and wanting to correct them.

So, we will assume that the fish is home in your tank and acclimated to the situation for the time being. The next question is, what about the other fish in the tank? While brackish species will tolerate freshwater fairly well for a period of time, freshwater species unfortunately do not handle brackish as well. So, if you have a brackish species mixed in with freshwater, they will need separate habitats. At this point, you must make a decision about whether you have the time, space, resources, inclination etc. to devote a new tank to the fish in question. If the answer is yes, then great! If not, then you should probably think about returning the fish to the store if you are unable to properly care for it.

Now that you have the fish and have decided you want to keep it, lets move on to the next step. Get the fish set up wherever it will end up, whether that’s the current tank or a new, cycled one. Now you can begin the process of converting it to brackish. The sensitive part of the system is not the fish at this point, but the bacteria in the filter. They are nowhere near as forgiving of salinity swings as the fish are. So, your goal is to slowly add salt to get the tank to the desired SG. The usual recommendation is raising the SG by .002 per week until you reach the desired SG. This will give the fish and the bacteria ample time to adjust to the change.

I have the tank set up, now what can I put in it?

There are many extremely interesting options for a brackish habitat, more than is initially apparent. I will break this up into a few different parts, so feel free to skip to the one that best applies to you. NOTE: These should not be viewed as stocking suggestions; rather, I would like to make people aware of what is available. I STRONGLY URGE you to do your own independent research as to water parameters required and compatibility issues for individual species you would like to house. It is also not an exhaustive list, there are many more species available than what are listed here. These are merely the most commonly found species in the aquarium trade.

Small tanks (30gal or less)

There are a few different species that are suitable for smaller tanks. Bumblebee gobies are an ever-popular option. They are cute and non-aggressive, and fairly easy to keep. A jade goby is another option, although I probably wouldn’t recommend them for anything much smaller than a 20 or 30gal tank. Some of the smaller puffers do well in species tanks of a 30gal minimum as well. There are some species of brackish moray eels that would also do well to start off in a smaller tank, but would eventually need to upgrade. They will also most likely require species tanks. Indian dwarf mudskippers are another cute alternative for a smaller tank. These fish require a certain specialization in that they need some land area in the tank. Do your research before purchasing one. Mollies also adapt well to brackish water, and will thrive and even breed in it long term. However, most store-bought mollies are kept and bred in alkaline fresh water, so the acclimation process will have to be slow.

Medium tanks (30-75ish gallons)

The options open up a little bit more with the larger tanks. In addition to everything that’s been mentioned, several other species of puffer fish are suitable to tanks this size. I am, however, no expert on puffer species. If you are interested in exploring further options in this vein, I would strongly recommend www.thepufferforum.com as an excellent resource. Most brackish morays will need a tank in this range as a long-term habitat. The Atlantic mudskipper is another very entertaining option for larger tanks, but again, please research its unique needs before going that route. The archerfish is another good possibility. This fish will get up to about 12” however, so a long term plan might include an upgrade to a tank closer to 90-100+ gallons. The violet goby fits into that category as well. They reach up to 20+ inches in captivity, but in my experience, are fairly slow growing. They are also fairly inactive.

BIG tanks (150gal+)

There are several other common species of brackish fish that aren’t really suitable for most aquarists. Monos and scats are two species that, while gorgeous fish, are pretty impractical for those new to brackish. These are schooling fish best kept in groups of 3 or more, and get up to a foot long. As a result, they shouldn’t be kept in tanks smaller than about 150 gallons, preferably larger. Columbian sharks are another tankbuster. These guys get 15-20”, so obviously need a lot of room to swim.

Inverts

Several invertebrates are suitable for brackish aquaria. Fiddler crabs and red-claw crabs are the first that jump to mind. These are also specialized species that require some land area to get out of the water. They should also not be kept with aggressive species; puffers as well as larger mudskippers will make meals out of crabs. Research indicates that nerite snails will also do well in low-end brackish tanks, but I have yet to find any to try it myself. I’ve also read that some species of ghost shrimp are native to brackish water, but most sold are the freshwater variety.

Plants

There are a variety of plants that will do well in low-end brackish. I have not had success with anything much above 1.008. Java fern and java moss both will do well. Some species of vallisneria will also thrive. For a more exhaustive list, look here.

Mangroves are another option for plants, but require a fair amount of work. They are light intensive, and can be delicate to acclimate when shipped. They also need regular dosing of magnesium to survive in brackish or marine environments. Also, they get huge – they are trees, after all. They are slow growing, however, so this might not be an immediate problem. I recommend extensive research before trying to grow these plants.